The overall objective is to investigate the origin, course, and termination of supraspinal descending pathways in the Tegu lizard (Tupinambis nigropunctatus). Reptilian species such as the one selected may be considered to be models of pure (i.e. without corticospinal influence) "extrapyramidal" organization. Their bulbospinal pathways are recognizably like those found in mammals, including man, yet they are sufficiently simplified that they offer tremendous opportunities for dissecting and understanding the function and structure of supraspinal motor control. More precise information about the morphological and functional organization of these pathways is of fundamental importance to understanding human health problems which involve the central nervous system control of motor acts, e.g. spinal cord injury and extrapyramidal syndromes such as Parkinsonism. Three series of experiments are proposed: (a) manipulations would be made in the spinal cord to cause a retrograde change (chromatolysis) or labeling (horseradish peroxidase) of the cell groups in the brain stem which project to the spinal cord, (b) in the specific brain stem cell groups identified above, manipulations would be made to cause anterograde changes (degeneration) or labeling (autoradiography) of their terminal fields in the spinal cord, and (c) the normal morphology of the spinal cord would be studied (using Nissl and Golgi stains and cobalt iontophoresis) in order to precisely define where the terminal fields are located and upon what postsynaptic structure the boutons terminate. In all experiments, material would be studied in both the light and electron microscopes.